COVERED-GILLED MOLLUSKS. 19% 
ORDER TECTIBRANCHIATA. 
(Covered-gilled Mollusks.) 
In general form and appearance, many of the 
animals contained in this Order resemble those of 
the preceding ; but they differ from them in having 
the gills attached along the right side, or upon the 
back. These organs assume the form of a single 
plume, or of leaflets more or less divided, but not 
symmetrical ; they are always more or less covered 
by the mantle, whence the name applied by Cuvier 
to the Order. The mantle encloses within its sub- 
stance, in almost all cases, a small shell to protect 
the vital parts. In some of the genera, the shell is 
developed to such a degree as to cover the animal ; 
as in the fresh-water limpet (Ancylus), found in some 
of our streams. For the most part, however, the 
species are marine ; they are widely scattered, but 
appear to be most numerous in the Indian and 
Mediterranean Seas. ‘The sexual functions are 
united in each individual, in which particular, this 
Order agrees with those which I have already con- 
sidered; but in the form of the breathing organs, it 
manifests a closer affinity with the following Order. 
The Covered-gilled Mollusca may be grouped in 
five families, three of which are represented by 
British species. 
FAMILY PLEUROBRANCHIDZ. 
‘The members of this family a cursory observer 
would at once associate with the Nudibranchs ; but 
